1. Field of the Invention
The present invention relates generally to antenna tracking systems, and more specifically to a satellite mobile communications system wherein a vehicle-mounted antenna is controlled to orient its bearing toward the satellite.
2. Description of the Related Art
In a conventional tracking system for a vehicle-mounted antenna, the orientation of the antenna is controlled in response to the difference between the antenna's bearing angle and the angle of arrival of a signal from the satellite so that the difference reduces to a minimum. This feedback operation continues as long as the antenna is in line of sight to the satellite. If the line-of-site to the satellite is obstructed by a land structure, the system enters an open-loop mode in which the vehicle's attitude is detected and used to control the antenna's bearing angle.
However, there is a no smooth transition as the system operation changes from the closed loop to open loop mode and then returns to the closed mode.
In addition, due to the vehicle's movements the bearing angle of the antenna cannot be precisely maintained within a desired range. Currently, the antenna's bearing angle has a tolerance of .+-.10.degree. to .+-.15.degree..